1. Field of the Invention
The invention relates to hard steel sheet manufactured from Al-killed continuous cast carbon-manganese steel. The invention also relates to a method for manufacturing such sheet, including the steps of continuously casting the steel, hot-rolling, cold-rolling, continuously annealing and skin-passing (cold finishing).
2. Description of the Prior Art
In this specification and claims, by the term steel sheet is meant a product which has been hot-rolled, cold-rolled, annealed and skin-passed and which has a thickness of 0.1 to 0.5 mm. Such a sheet may additionally be provided with a metallic surface-layer such as for example tin or chrome/chromic oxide (ECCS) or with a chemical surface layer such as lacquer. Steel sheet is obtainable in various hardness categories. The softer qualities of sheet are used when, in manufacturing a product therefrom, the deformation given to the sheet is large, for example in the manufacture of certain cans. The harder qualities of sheet find use when the deformation to which the sheet is subjected to less large and strength requirements are set, such as for example with can ends.
The present invention aims for example particularly at the production of sheet in the hardness categories T61, T65 and T70 of European Standard 145-78 which is sheet with a hardness HR30T of 57 and higher. The mean hardness HR30T and the range permitted in these categories are as follows:
______________________________________ Hardness HR30T Hardness category Mean Range ______________________________________ T61 61 .+-.4 T65 65 .+-.4 T70 70 +3 -4 ______________________________________
HR30T is the Rockwell hardness using the 30T Rockwell Scale.
In other Standards, such as Tin Mill Products May, 1979 of AISI (American Iron and Steel Institute) and JISG 3303 (1984) of the Japanese Institute of Standards, other hardness-category designations are given, and there are slight deviations from the ranges of European 145-78 specified above. However grades of sheet defined in such other standards are deemed to satisfy European Standard 145-78 when the mean hardness-value HR30T corresponds to one of the categories T61, T65 and T70, and the present invention extends to these corresponding grades.
There are two known methods of producing hard qualities of sheet. The first method consists in that by skin-passing a great reduction of the thickness up to 15% of the thickness before skin-passing is obtained, the material being strengthened thereby. This has not only the disadvantage that a severe skin-passing is required but also that after skin-passing the steel sheet is more anisotropic, due to variations of mechanical properties between the direction of rolling and the direction at right angles thereto, than is the case when in the skin-passing a smaller reduction in thickness is performed. This anisotropy can be serious when the steel is subsequently subjected to, for instance, deep-drawing or pressing.
The second known method consists in that a higher carbon and manganese content is used in the chemical composition of the steel than for the softer steel qualities. This makes the steel sheet harder and stronger, but a disadvantage is that steel with a higher carbon and manganese content is more expensive and offers greater resistance to deformation during cold-rolling and skin-passing. Yet another disadvantage is that different chemical compositions are needed for different hardness categories, so that a manufacturer cannot start from a standard steel suitable for a range of qualities.